This invention relates to controlling the separation of boundary layer flow by means of the combination of a small protrusion just upstream of the expected separation point and dynamic forcing, through a slot at the downstream base of the protrusion, in response to harmonically and phase related forcing signals.
Loss of efficiency and performance as a consequence of boundary layer flow separation is known to cause retreating blade stall, drag of bluff bodies, jet engine air inlet distortion, as well as losses in diffusers, heat exchangers and pumps in heating, ventilating and air conditioning systems. Known separation controls have included passive vortex generators as well as blowing or suction in the near-wall regions where separation may occur. A recent approach that causes the boundary layer to absolutely attach to the surface is disclosed in commonly owned U.S. patent application Ser. No. 09/257,565, filed on Feb. 25, 1999, which achieves clearly superior results with significantly low levels of control energy input. However, there is a possibility that restrictions on the amount of control that can be practically provided in certain applications may render that approach less useful in those certain applications.
Objects of the present invention include provision of a flow boundary layer separation control with smaller parasitic drag, and requiring less energy than prior forcing techniques.
This invention is predicated partially on the realization that the unsteady nature of flow structures in and near a boundary layer result in motion of the separation point in an uncontrolled flow, whereby a point of receptivity to a control impetus is not well defined, nor static, which makes it difficult to position the control device correctly and thereby endangers the effectiveness of the control when any change in flow conditions may cause the separation point to migrate a functionally significant distance from the control location.
According to the present invention, the separation of a flow boundary layer is controlled by inducing a flow separation of thickness on the order of the thickness of the boundary layer, such as by means of a step or other geometrical protrusion, and an oscillatory fluid pressure jet inlet (downstream-facing) in the wall surface immediately downstream of the protrusion, the inlet providing alternating sucking and blowing in response to a combination of frequencies which include a fundamental frequency and at least one subharmonic frequency, said frequencies related by selected phase angles. In one embodiment, the jet excitation signal may take the form
A sin[2xcfx80(f/3)t]+B sin[2xcfx80(f/2)t+xcfx861]+C sin[2xcfx80(f/3)t+xcfx862]xe2x80x83xe2x80x83EQN. 1
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.